1. Field of the Invention
The present invention relates to a combustion-type power tool, such as combustion-powered fastener-driving tool for driving fasteners, such as nails, into a workpiece.
2. Description of the Related Art
Compressors and hoses as required in conventional compressed-air type power tool are not needed in combustion-type power tools and the combustion-type power tools are good in operability. Various proposals have been made with respect to the combustion-type power tools as disclosed in U.S. Pat. Nos. 5,197,646 and 4,483,474.
Unlike a compressed-air type power tool that uses compressed air as a driving source, the combustion-type power tool requires no compressor and is, therefore, much easier to transport to a construction site or the like. Further, the combustion-type power tool includes an internal power source, such as a battery, so that the tool can be used in any environment without requiring a commercial power supply.
FIG. 6 is a cross-sectional view showing a conventional combustion-type nail gun 101.
In the combustion-type nail gun 101, a cylinder 105 is fixedly disposed within an outer frame 102. A piston (not shown) is slidably movably disposed within the cylinder 105. A combustion chamber frame 119 is disposed to surround the cylinder 105 and movable in the direction in which the piston moves. The outer frame 102 is partitioned by a partition wall 103 into spaces S1 and S2. The cylinder 105 and the combustion chamber frame 119 are accommodated in space S1, and a gas cartridge cylinder 122 in space S2. A through-hole 103a is formed in the partition wall 103, allowing spaces S1 and S2 to be in fluid communication with each other. A combustion chamber frame holding rod 125 is generally disposed within space S2. One end of the rod 125 is inserted into the through-hole 103a and projected into space S1 so that the end of the rod 125 engages the lower potion of the combustion chamber frame 119. The combustion chamber frame holding rod 125 moves in cooperation with a trigger switch 124 and serves to hold the combustion chamber frame 119 when the trigger switch 124 is turned off.
In use, the nail gun 101 is moved downward toward a workpiece W from the state shown in FIG. 6. When a push lever 121 is brought in abutment with the workpiece W and pushed thereagainst, the push lever 121 moves upward against the biasing force of a spring 120 biasing the push lever 121 downward. The combustion chamber frame 119, which is coupled with the push lever 121 via an arm 131, is also moved upward. In this manner, the combustion chamber frame 119 moves upward along the cylinder 105. Raising the combustion chamber frame 119 to the uppermost position forms a hermetically sealed combustion chamber S by a cylinder head 104, the combustion chamber frame 119, the cylinder 105, and the piston. Specifically, the combustion chamber S is formed by the engagement of the upper inner periphery of the combustion chamber frame 119 with the cylinder head 104, and the engagement of the middle inner periphery of the combustion chamber frame 119 with the cylinder 105. Flammable gas stored in a gas cartridge cylinder 122 is injected into the combustion chamber S. The flammable gas is agitated and mixed with air in the combustion chamber S by a fan 115. A spark plug exposed in the combustion chamber S produces a spark for igniting and burning the gaseous mixture. The combusted gas expands to move the piston downward. A driver blade (not shown) secured to the piston strikes the nail into the workpiece W. A push switch 132 is provided for detecting that the combustion chamber frame 119 has elevated to a predetermined position.
When the piston has downwardly moved to a position near the lower dead center, an exhaust hole formed in the cylinder 105 is open to the atmosphere. High temperature, high pressure combusted gas in the cylinder 105 is discharged out to atmosphere through the exhaust hole and a stop valve disposed in the exhaust hole. Then, the pressure in the combustion chamber S is gradually lowered. When the pressure in the combustion chamber S has reached atmospheric pressure, a check valve is closed to thereby hermetically seal the combustion chamber S. Thermal vacuum caused by rapid cooling of the combustion chamber S draws the piston back to its initial upper dead center shown in FIG. 6.
The user subsequently lifts the nail gun 101 so as to be separated from the workpiece W. When the user releases the trigger switch 124 (turns the trigger switch 124 off), the combustion chamber frame holding rod 125 is disengaged from the lower portion of the combustion chamber frame 119. Due to the biasing force of the spring 120, the combustion chamber frame 119 returns to the initial position shown in FIG. 6. Therefore, the combustion chamber S is not hermetically sealed but is open to atmosphere. In this state, a motor 113 has been driven by a control circuit (not shown) and thus a fan 115 continues rotating. The rotating fan 115 draws fresh air through an inlet 112a formed in a cylinder head 112. The fresh air is introduced into the combustion chamber S through a flow channel 128, thereby performing a scavenging operation in which the fresh air introduced into the combustion chamber S expels the exhaust gas remaining in the combustion chamber S.
Continuous nail driving operations with the conventional nail gun 101 accumulate heat generated when the flammable gaseous mixture is combusted, resulting in heating up the nail gun 101, particularly the combustion chamber frame 119 and the cylinder 105. In the scavenging operation performed after the nail driving operation, these heated-up members are cooled. It should be noted that the exhaust gas primarily flows toward the lower portion of the combustion chamber frame 119 and is discharged out to the combustion chamber frame 119 through a discharge port 119a and then out to the outer frame 102 through an opening 130.
The through-hole 103a formed in the partition wall 103 allows a part of high temperature exhaust gas to pass therethrough. That is, the high temperature gas existing in space S1 in which the cylinder 105 and the combustion chamber frame 119 are accommodated flows into space S2 in which the gas cartridge cylinder 122 is accommodated. As a result, the gas cartridge cylinder 122 is heated up, causing the temperature of the gas cartridge cylinder 122 to increase.
The pressure of the fuel confined in the gas cartridge cylinder 122 changes greatly depending upon the change in temperature. Accordingly, the temperature rise of the gas cartridge cylinder 122 causes a fuel ejection amount to vary and so a constant amount fuel ejection is not ensured. With the combustion-type nail gun 101, the gaseous mixture in the combustion chamber S can be ignited only when the density of the flammable gas is within a predetermined range. If the density of the flammable gas is too low or too high to be outside the predetermined range, the gaseous mixture may not be ignited. Even if the gaseous mixture could successfully be ignited, the output power would be dramatically reduced, prohibiting stable performance of the nail driving operation.